1. Field of the Invention
The present invention relates generally to protocol analyzer pods, and more particularly to protocol analyzer pods for the Integrated Services Digital Network (ISDN) U-interface.
2. Related Art
The Integrated Services Digital Network (ISDN) has a number of access points, each with differing signals and data rates. A U-interface is one of these access points. The ISDN U-interface standard is described in a document entitled ANSI T1.601-1988, which is herein incorporated by reference in its entirety.
The U-interface is intended for use with an ISDN Basic Rate Access. The ISDN Basic Rate Access includes two 64 Kbit/S full duplex data channels (called B1 and B2) and one 16 Kbit/S full duplex control channel (called D). The two data and one control channels (abbreviated 2B+D) yield a rate of 144 Kbit/S full duplex for the ISDN Basic Rate Access. The two data and one control channels, plus 16 Kbit/S of overhead information, make up a 160 Kbit/S U-interface data stream.
The 160 Kbit/S U-interface data stream is sent and received full duplex over unloaded standard telephone twisted pair up to 18,000 feet. To reliably send and receive data at this rate over standard telephone twisted pair requires very complex U-interface transceivers. These U-interface transceivers use complex digital signal processing algorithms for adaptive equalization and digital echo cancelling techniques. A number of modulation/demodulation schemes are used. These modulation/demodulation schemes are called line codes. The line code most common in America today is called AMI (Alternate Mark Inversion). However, the standard that most operating companies are moving to is called 2B1Q (2-Binary-1-Quaternary). The 2B1Q line code is also called the ANSI line code. U-interface transceivers have been built for both of these line codes as well as others.
There are problems involved with monitoring a U-interface line wherein U-interface line codes (such as AMI and 2B1Q) that send and receive full-duplex are being used. First, a first signal that is being sent in one direction can be received only if a second signal that is being sent in the opposite direction is known. This is true in both directions. Second, adaptive equalization cannot be performed over the entire U-interface line. Rather, adaptive equalization is effective only at one point on the U-interface line. Therefore, in the general case, the U-interface line cannot be monitored without knowledge of the data on the U-interface line.
A prior solution to this problem involves making assumptions regarding the data on the U-interface line. However, such assumptions can be made only in special situations. This prior solution is very restrictive and cannot be guaranteed for all situations.
Therefore, a system and method is required for monitoring U-interface data streams in a communication system wherein full duplex U-interface line codes (such as AMI and 2B1Q) are used.